The format behind TLS, code signing and machine identity, explained without the ASN.1.
An X.509 certificate is a signed statement: this public key belongs to this identity, and I — the issuer — vouch for it until this date. Everything else is detail. A certificate carries the subject, the public key, the issuer, a validity window, and extensions describing what the certificate may be used for. A certificate authority signs it, and your system trusts the result because it already trusts the root at the top of the chain.
Validation levels describe how hard the issuer looked before signing. Domain validation proves control of a hostname and can be fully automated. Organisation and extended validation involve checking a real legal entity, which is why they cost money and take days. None of the three changes the encryption.
The clearest way to see it work: request a domain-validated certificate over ACME and watch the chain build.
To understand the issuer side, run your own CA and issue one yourself.
Where organisation and extended validation are actually worth paying for.
If you remember one thing: the certificate is a signed claim with an expiry date, and the entire discipline is making sure that date never surprises you.
Describe the estate — roughly how many certificates, public or internal, and what has to be automated — and we’ll send back a shortlist with the reasoning. No vendor sees your details.